SVGHMI: rework Jump widget to cope with Enable Expressions.

Relative jumps are not disabled implicitely anymore when pointed HMI_NODE is 0.
Also, "activable()" implementation was enhanced to make it optional.

          <xsd:attribute name="Command" type="xsd:string" use="optional" default="make -C %(buildpath)s all BEREMIZSRC=%(src)s BEREMIZCFLAGS=%(cflags)s MD5=%(md5)s USE_BEREMIZ=1 FROM_BEREMIZ=1"/>